The present invention generally relates to remote telephone switching and testing and, more particularly, to apparatus which provides a customer who leases a private telephone line from a telephone company with circuit test capability independent of telephone company provided service.
Private telephone lines are leased from the telephone company to provide data transmission or the like between a first customer premise and one or more second remote customer premises. The private telephone line is classified as voice grade four-wire service, the four wires being a receive pair (tip and ring) and a transmit pair (tip and ring). This four-wire service passes through at least one telephone company central office, and at each customer premise, a terminating unit terminates the respective four-wire line and provides a connection to customer installed equipment.
Telephone company service and maintenance frequently does not extend beyond the terminating units for the four-wire lines at each of the customer premises. This telephone company service includes certain tests which are routinely performed or performed for diagnostic purposes. Typically, the terminating units at the customer premises are provided with loop-back test circuits which are responsive to a tone having a frequency of 2713 Hz. This tone is generated at the telephone company central office, and when it is received by the loop-back test circuit at one of the terminating units, the loop-back test circuits function to simultaneously disconnect customer equipment and provide a connection from the receive tip to the transmit tip and a connection from the receive ring to the transmit ring. This connection from the receive pair to the transmit pair is made with a loop-back amplifier which raises the -16 dBm received test signals to 0 dBm providing equal level loopback. Receipt of a looped back signal at the telephone company central office provides confirmation of the loop-back condition of the four-wire line under test, thereby providing an initial continuity test. A milliwatt test signal source providing a precision 1004 Hz sinusoidal signal at 0 dBm may then be connected to the four-wire service at the telephone company central office. This signal propagates through the loop established by the loop-back condition, and the signal received back at the telephone company central office is used to test the loss, distortion and noise of the four-wire system. Once these tests have been completed, the four-wire line is returned to service by a second tone received by the loop-back test circuits which then reconnect the customer equipment to the four-wire line.
The telephone company test procedures have the advantages of being quickly performed from a central location without the need of sending a technician to the customer's premise. As a result, service disruption and expense are minimized. However, these tests are performed only on the telephone company four-wire service and do not extend beyond the terminating units on the customer's premises. Moreover, the tests are typically made only during the telephone company's regular business hours, in which case disruption could mean a loss of several hours service to the customer until the opening of the next business day.
As a specific example, consider the case where the remote customer premise includes a plurality of data collecting and/or processing equipment which are connected by means of modems (modulation and demodulation units) for connection to the telephone line. In this specific example, connection of the several modems to the four-wire line is made by means of distribution and collection bridge pairs, also known as split data bridges. This equipment may be associated with a data processing center, a manufacturing facility or the like which may itself be unmanned at least part of the time. In that case, the telephone company provided service will not test each of the several channels which connect the individual modems to the four-wire line because of inability to discriminate between channels beyond the bridge. Even if the telephone company were to provide a test of the individual channels connecting the modems to the four-wire service, the type of test normally performed by the telephone company from the central office cannot measure the two-wire sine wave return loss of the transmit channels emanating from the respective modems, without access to the remote end of those channels. Yet another problem faced by the customer is the possibility of a malfunction of one of the data collection and/or processing equipments at the remote location which results in a condition known as "data-streaming." In this condition, the equipment puts out a continuous stream of noise which is generally invalid data, but since the data stream is continuous, that equipment effectively seizes the telephone line, preventing the transmission of valid data from other equipments over the telephone line. If this condition persists, the customer may lose hours of private line service. It is therefore desirable to disconnect the malfunctioning equiment from its channel, thereby freeing the telephone. line for use by the other channels. The malfunctioning equipment can then be serviced at the customer's convenience.
Since the customer is generally the first to recognize service failures, the customer should be in a position to independently test his service and ascertain the nature of service failures in terms of the parameters tested. Thus, when the customer's service call is received by the telephone company, the customer is in a position to submit test results as well as request the service, thereby expediting the return of full service to the customer. What is needed, therefore, is a remote telephone line switching and testing apparatus which will facilitate testing service between one location and at least a second remote location.
According to the invention there is provided a remote telephone line switching and testing circuit for a four-wire communication system providing communication between equipment at a first location and equipment of at least a second remote location. Typically, the four-wire communication system includes one, and sometimes more, telephone company central offices connected to the first location and the second remote location by respective four-wire lines and providing a communication path therebetween. The switching and testing circuit is adapted to be connected to the receive pair at the remote location and includes a tone detecting circuit which is responsive to tones of a predetermined frequency for providing output detection signals. A control circuit is responsive to those output detection signals to generate control signals.
According to one aspect of the invention, switching circuitry is provided which is responsive to a first control signal from the control circuit for disconnecting the equipment at the second location and providing quiet terminations for both the receive and transmit pairs of the four-wire line. This is the data-streaming dropout mode. The switching circuitry is further responsive to a second control signal from the control circuit for re-establishing the connection between the equipment at the second location and the four-wire line.
According to another aspect of the invention, switching circuitry is provided which is responsive to a first control signal from the control circuit for disconnecting the equipment at the second location from both pairs of the four-wire line and connecting a milliwatt test signal source to the transmit pair. The switching circuitry is further responsive to a second control signal from the control circuit for disconnecting the milliwatt test signal source from the transmit pair and reconnecting the equipment at the second location to the four wire line.
In the preferred embodiments of the invention, the control circuit is a sequencer circuit including a two-stage binary counter and decoding logic. In the first preferred embodiment, the counter counts three detection signals and is then automatically reset after a brief delay. On the first count output, the switching circuitry disconnects the equipment at the second location from the four-wire line and connects a loop-back amplifier between the receive pair and the transmit pair. On the second count output, the switching circuitry disconnects the loop-back amplifier and provides quiet terminations for each of the receive and transmit pairs. On the third count output, the loop-back amplifier is briefly reconnected between the receive pair and the transmit pair to provide a brief tone pulse, and thereafter the loop-back amplifier is disconnected and the equipment at the second location is reconnected to the four-wire line. At this time the counter is reset to be ready for the next cycle of switching and testing.
In the second preferred embodiment, the counter counts four detection signals, and since this count results in both stages of the counter being in their initial states, there is no need to provide a reset for the counter. On the first count output, the switching circuitry disconnects the equipment at the second location from the four-wire line and connects a loop-back amplifier between the receive pair and the transmit pair. On the second count output, the switching circuitry disconnects the loop-back amplifier and provides quiet terminations for each of the receive and transmit pairs. On the third count output, a milliwatt test signal source is connected to the transmit pair. On the fourth count output, the milliwatt test signal source is disconnected and the connection of the equipment at the second location to the four-wire line is re-established.
In the case of the specific example mentioned earlier where the second location includes a plurality of modems, a separate switching and testing circuit can be provided for each modem. Each switching and testing circuit is assigned a different predetermined tone frequency to which only its tone detecting circuit is responsive. With this arrangement, it is possible to separately address each switching and testing circuit to provide a multipoint testing capability.
The tone generator to which the tone detecting circuit is responsive can be at any convenient location which in this application will simply be referred to as the first location. Those skilled in the art will understand that this first location could be a first customer premise or any intermediate point between a first customer premise and a second remote customer premise including a telephone company central office. If the multipoint testing capability is provided, the tone generator is a multi-tone generator with any one of the predetermined tone frequencies being selectable. To prevent activation of the tone detecting circuits by data, the activating tone signal is applied nominally 10 dB higher than data level, and the tone detector sensitivity is reduced to disregard data level signals.
The invention is generally useful in any four-wire communication system and provides a flexibility in switching and testing between any pair of locations in the system not heretofore available.